Scarriffhollis, an Scairbh Sholais, "the shallow of light"/"the light shallow", is a ford in Donegal near Letterkenny.
It is the place of a battlefield with a desastrous defeat in 1650, known in Irish as "Maidhm na Scairbhe Soilse"

But why does the genitive "Sholais" (of light) change to "Soilse" ([of] lights)?

"Soilse" could also be the gen sing of soilse fem. - 'brightness, light', but that doesn't answer your question. Hmm...interesting.